Liquid toner comprises toner particles dispersed in a carrier liquid. Printing an image on a substrate using liquid toner involves extracting toner particles from the carrier liquid and depositing the extracted toner particles on the substrate in a pattern suitable to form the image. Once deposited on the substrate the particles are bonded together and to the substrate to provide the finished image.
Toner particles are charged and electric fields are used to transfer toner particles from the carrier liquid and deposit them on the substrate. Generally, the toner particles are dispersed in relatively low concentration in the carrier liquid and printing with liquid toner usually begins with an electrophoretic process that increases toner particle concentration in toner used to print an image.
In some liquid toner printing systems, concentration of toner particles used to form an image is accomplished by a system, hereinafter referred to as an “application system”, that comprises a cylinder formed from a conducting material and an associated electrode positioned near to the surface of the cylinder. The cylinder is hereinafter referred to as an “applicator” and the electrode is referred to as a “concentration electrode”. The concentration electrode has a surface facing the applicator that is a portion of a cylinder having its axis with the axis of the applicator. This surface of the concentration electrode and the surface of the applicator form a narrow uniform space, hereinafter referred to as a “application space”, between the concentration electrode and the applicator. A projection of the surface of the concentration electrode onto the surface of the applicator shadows a relatively narrow area on the surface having a length substantially equal to the length of the applicator. The application space has an inlet aperture and an outlet aperture substantially parallel to the axis of the applicator. The applicator is rotated about its axis and toner is pumped from a suitable reservoir so that it enters the application space through the inlet aperture, flows through the application space and exits the application space through the outlet aperture.
A voltage difference is applied between the applicator and the concentration electrode to generate an electric field, hereinafter referred to as an “application space electric field”, in the application space. The direction of the electric field in the application space is such that toner particles in toner flowing through the application space migrate towards the surface of the applicator. As the applicator rotates, regions of its surface “enter” and pass through the application space passing by the concentration electrode. As a surface region of the applicator passes through the application space, toner particles in toner flowing in the application space migrate to and adhere to the surface region. When the surface region leaves the application space it is covered with a thin layer of toner in which the concentration of toner particles is substantially greater than that of toner particles in toner in the reservoir. A squeegee roller contacts the region after it leaves the application space and removes excess carrier liquid from the toner layer. A voltage difference is maintained between the squeegee and the applicator that repels toner particles from the squeegee and enhances concentration of toner particles. Concentration of toner particles in the toner layer covering the surface region after it is “squeegeed” is determined by the voltage differences between the applicator and the concentration electrode and between the applicator and the squeegee.
The applicator contacts a photoconducting surface on which an electrographic image corresponding to the image to be printed is formed using methods and devices known in the art. Generally, the photoconducting surface is a surface of a cylinder, hereinafter referred to as an “imaging roller”, that rotates in a direction opposite to the direction of rotation of the applicator. The applicator rolls on the imaging roller and transfers toner particles to regions of the imaging roller responsive to voltage differences between the applicator and the electrographic image on the imaging roller. In some printing systems toner from the imaging roller is transferred directly to the substrate to form the image. In other imaging systems, toner from the imaging roller is transferred to an intermediate transfer member, which in turn transfers the toner to the substrate. Perceived quality of the printed image depends, inter alia, on uniformity of a layer of toner particles deposited on printed regions of the substrate and the density, hereinafter referred to as “area density”, of toner particles in the layer per unit area of printed substrate.
Liquid toner printing systems in which toner particles are extracted from liquid toner using an application system of the type described above and transferred to a substrate to print an image on the substrate are described in U.S. Pat. Nos. 5,596,396 and 5,737,660, the disclosures of which are incorporated herein by reference.